The enantioselectivehydrogenation of (E)-2,3-diphenylpropenoic acids over Pd/Al2O3 modified by (R)C8(S)C9 cinchona ethers resulted in the inversion of the sense of the enantioselectivity. To find the explanation of the phenomenon, the interaction of acids bearing different substituents with cinchona alkaloids was studied in solution by NMR spectroscopy and experiments using mixtures of modifiers were
(R)C 8(S)C 9修饰的Pd / Al 2 O 3上的(E)-2,3-二苯基丙酸的对映选择性氢化金鸡纳醚导致对映选择性的反转。为了找到现象的解释,通过NMR光谱法在溶液中研究了带有不同取代基的酸与金鸡纳生物碱的相互作用,并进行了使用改性剂混合物的实验。与母体生物碱相比,获得的突出的非线性行为表明金鸡纳甲基醚的吸附发生了变化。研究表明,醚衍生物与不饱和酸的相互作用更加灵活,并且甲基的存在改变了手性表面的位置。这些效果的组合,再加上二芳基取代的丙烯酸的庞大体积,可能会导致底物在吸附的改性剂手性口袋改变后的对接偏好发生逆转,从而导致对映选择性降低,甚至导致对映体的逆转。它的意义。关于()对映选择性氢化中配体加速机制的新证据还提出了在苄胺存在下在金鸡纳生物碱改性的Pd上的E)-2,3-二苯基丙酸。
Up to 96% Enantioselectivities in the Hydrogenation of Fluorine Substituted (E)-2,3-Diphenylpropenoic Acids over Cinchonidine-Modified Palladium Catalyst
High enantioselectivities, up to 96%, were obtained in the hydrogenation of some disubstituted derivatives, unprecedented in the hydrogenation of prochiral unsaturated carboxylicacidsovermodified heterogeneous catalyst. The best optical purities were reached in the hydrogenation of derivates bearing a para-substituent on the β phenyl and an ortho-substituent on the α phenyl ring, respectively. The
enyl)propenoic acid showed 92%ee with an activity increase of 230%. It is considered that the difference is caused by the ortho-methoxy substituent on the α-phenyl ring. The substituent effect of the ortho-substituent on the α-phenyl ring demands the presence of an electron-donor substituent on the β-phenyl ring. A remarkable substituent effect was observed with 3-(p-methoxyphenyl)-2-(o-methoxyphenyl)propenoic
Asymmetric transfer hydrogenation of 2,3-diphenylpropenoic acids over heterogeneous palladium catalysts modified by cinchona alkaloids
作者:György Szőllősi、Vivien Erzsébet Resch、Vanessza Judit Kolcsár
DOI:10.1016/j.jcat.2024.115290
日期:2024.1
supported palladium catalysts modified by dihydrocinchonidine carried out in ordinary laboratory glassware using easily available, cheap hydrogen donor, potassium formate. High conversions and good enantioselectivities were obtained in transfer hydrogenations of various acids substituted on phenyl rings. Results obtained with various acid derivatives and with few cinchona alkaloid modifiers indicated that
Relationship between substrate Structure and enantioselectivity is studied for the asymmetric hydrogenation of 42 different (E)-alpha, beta-disubstituted acrylic acids (propenoic acids) over cinchonidine-modified Pd/C. The beta-phenyl group is indispensable for high enantioselectivity of alpha-phenylcinnamic acid (2,3-diphenylpropenoic acid, 81% ee), and substitution on this group affects markedly the selectivity. The high ee up to 92% was achieved by the beta-p-alkoxyphenyl substitution, and the selectivity is ascribed mainly to stronger interaction of the substrate with the chiral modifier on the catalyst surface. In contrast, substitution on the alpha-phenyl group does not affect notably the enantioselectivity (80-82% ee) or even the alpha-phenyl group itself is not indispensable but replaceable with a properly bulky group for the high enantioselectivity, (C) 2008 Elsevier Inc. All rights reserved.